The present invention relates to means for heating particulate material and especially minerals that expand upon heating. The invention finds particular utility in expanding particulate perlite.
Perlite is a siliceous material of volcanic origin having a silicon content of about 65 percent by weight, a combined water content of from 2 to 5 percent by weight and additional amounts of aluminium, sodium, and potassium. Perlite is mined in a strip mining operation where the over burden is removed and the perlite ore collected. The ore is crushed and then heated to a temperature in the range from 870.degree. to 1150.degree. C. When the material is heated in that range the mineral is softened and suddenly expands to form a light-weight cellular particle when the water within the particle expands. The expanded cellular particle is relatively strong, inert, and resistant to relatively high temperatures. As a result, it finds application in a number of industrial products including filter aids, thermal insulation, inert fillers for such materials as plaster and concrete, and other applications requiring a light-weight inert particulate material. In most of these applications the expanded perlite is utilized in large quantities as a bulk product. Because of the relatively large amount of energy associated with the expansion process there has been a long standing effort to develop efficient and practicle means of carrying out the expansion.
In addition to the expansion of perlite, there are number of industrial processes that require the heating of particulate materials to high temperatures. For example, there are other mineral products that can be expanded by drying or the application of heat such as vermiculite, some types of clay, shale, obsidian. Even organic materials such as cereal grain may be expanded by a high temperature treatment, albeit at a different temperature range than mineral products.